An aerosol is a suspension of fine solid particles or liquid droplets in a gas. Aerosols that are present in the earth's atmosphere affect propagation of electromagnetic radiation in at least the visible and near infrared parts of the spectrum. Many uses of images taken by remote sensors require that the effects of atmospheric scattering and absorption be taken into account so that the true radiance leaving the ground can be determined.
There are a number of applications for methods of determining the characteristics of the aerosols at a particular location, one of which is remote material identification. Remote material identification processes use specially programmed computers to analyze and determine from images of a remote object the material from which its surface is made. The determination is based in part on the spectral radiance of the object's surface within the image. The target's spectral reflectance is estimated from the measured spectral radiance, using a model of the illumination of the target surface by the sun and atmosphere. Once the spectral reflectance of the target material is determined, it can be compared with the known spectral reflectance of different materials for a match.
Because aerosol in the atmosphere will affect the propagation of the electromagnetic radiation, it should be taken into account by the model when determining the reflectance of the target material. Examples of parameters typically used to characterize aerosols include extinction coefficient, absorption coefficient (or equivalently by a single scatter albedo), and asymmetry factor. Background is characterized by a single band-effective reflectance in each sensor band.
One approach to inferring aerosol properties from multispectral images is to assume an aerosol type, and then attempt to characterize the amount of that aerosol present in the atmosphere. For example, the Regression Intersection Method for Aerosol Correction (RIMAC) assumes an aerosol type and then proceeds to use image data to estimate the visibility, which is directly related to aerosol concentration. See Sanders, “An Atmospheric Correction Algorithm for Hyperspectral Imagery”, Doctoral Dissertation, Rochester Institute of Technology, 1999. In another prior art approach, the land aerosol retrieval algorithm for the Moderate Resolution Imaging Spectroradiometer (MODIS) assumes a mixture of two types of aerosols (a coarse aerosol and a fine aerosol), determines the mixing ratio, and then estimates the aerosol optical depth, which is directly related to aerosol concentration. See, Remer et al, “Algorithm For Remote Sensing Of Tropospheric Aerosol From MODIS: Collection 5”, Algorithm Theoretical Basis Document, http://modis.gsfc.nasa.gov/data/atbd/atmos_atbd.php.
An in-scene method for estimating target reflectance from spectral reflective imagery, called Empirical Line Method (ELM), takes into account atmospheric conditions. However, it requires the use of at least two, and preferably three or more, calibration references of known reflectance be present in the scene.